Organic substrates, for example printed circuit boards and chip carriers, have been and continue to be developed for many applications. They are expected to displace ceramic chip carriers in high-end applications because of enhanced performance.
The use of organic substrates introduces new manufacturing challenges because of the inadequacy of existing manufacturing infrastructure. The tools required for the manufacture of organic chip carriers are notably different than those required for either ceramic chip carrier products or printed circuit boards.
The article "High Performance Carrier Technology: Materials and Fabrication," written by D. N. Light, F. J. McKiever, C. L. Tytran, and H. L. Heck, which was presented at the 1993 International Electronic Packaging Conference and published in Int. Electronic Packaging Society, Wheaton, Ill., provides an exemplary description of some the materials used for a substrate design and is incorporated herein by reference. Ceramic filled polytetrafluoroethylene (PTFE) (e.g., Rogers 2800.TM.) is a leading candidate material for organic chip carriers because of good dielectric properties (E.sub.r =2.8) and high temperature capability (T.sub.melt =327 C.). This type of dielectric material is available in very thin layers (1.0 mil) which further enhances the packaging density. In addition, the compatibility of PTFE with high temperature processing permits the use of known high yield ceramic processes to form circuitry and solder dams and to attach chips. PTFE also provides a thin, low inductance cross-section with impedance matching and enables reliable via structures on grids of 10 mils or less. Yet another benefit of PTFE is a low modulus of elasticity which reduces the strain imposed on chip attach solder joints during thermal cycling of the chip-substrate assembly.
Commercial considerations make it desirable to use existing board manufacturing capability in combination with existing ceramic substrate line metallization capability to produce "hybrid" organic substrates. For example a PTFE based chip carrier which includes Rogers 2800.TM. material as the dielectric in the base laminate can readily have the top surface metallization done in a substrate metallization line, thus forming a package that is similar to existing ceramic packages with respect to chip assembly.
Unfortunately, the low modulus of elasticity of PTFE creates a handling challenge during the manufacture of PTFE based laminates. When compared to an equivalent thickness cross-section of a common glass-cloth reinforced dielectric such as FR-4, a PTFE based organic substrate core is an order of magnitude more compliant making it easy to deform, wrinkle, and damage during processing. Lack of flatness at assembly can also be a problem.
To control the coefficient of thermal expansion (CTE) and to improve stiffness for processing, 6 mil copper clad Invar alloy has been used at the center of the hybrid organic substrate core. (Invar is a registered trademark of IMPHY S.A., Paris, France for an "alloy which is substantially inexpansable.") Invar is an iron-nickel alloy containing approximately sixty four weight percent iron and thirty six weight percent nickel. Although Invar alloy has a relatively high modulus of elasticity, it provides minimal enhancement to overall stiffness of an organic substrate when placed at the center of the cross-section, (i.e., at the neutral axis for bending). The result is a substrate core with minimal bending stiffness and undesirably low flexural yield strength.
Although glass cloth reinforcment of the dielectric can enhance flexural rigidity and handling characteristics, it can also degrade package reliability by introducing risk sites for insulation resistance failure. Interfaces along cloth fibers can form paths of conductance between adjacent conductors in the carrier which can result in ionic migration and insultation resistance failure of the package.
An organic substrate for electronic components should have mechanical integrity and not be fragile nor tear, stretch, or wrinkle during processing. For best performance, the final cross-sectional thickness preferably should be approximately 10 to 15 mils.